In recent years, a package substrate for connecting IC chip or the like to a mother board or a daughter board have been required to reduce its dielectric constant and dielectric loss factor because the frequency of each signal has been raised. Therefore, the mainstream of the material of the substrate has been changed from ceramics to resin.
In the foregoing background, as a technique relating to a printed circuit board incorporating a resin substrate, a so-called build-up multilayer printed board has been suggested in Japanese Patent Publication No. 4-55555. That is, epoxyacrylate is used to form an inter layer resin insulating layer on a glass epoxy substrate having formed circuits. Then, a photolithographic method is employed to form an opening for a via hole. The surface is coarsened, and then a plating resist is provided to perform a plating process so that a conductive circuit and the via hole are formed.
When the foregoing build-up multilayer printed circuit board is employed as a package substrate, a conductive connecting pin for establishing the connection with a mother board or daughter board must be joined.
The foregoing pin is called a “T-type pin” having a T-like side shape realized by a columnar connecting portion 722 and a plate-like secured portion 721 as shown in FIG. 76. Thus, the connecting portion 722 is used to establish the connection with a socket of mother board or the like. The foregoing conductive pin 710 is, with a conductive adhesive agent 717, such as solder, bonded and secured to a pad 716 which is a conductive layer of an inter layer resin insulating layer 752 (or a core substrate) which is the outermost layer of the build-up multilayer circuit plate.
The foregoing structure suffers from an excessively small area of contact between the pad 716 and the inter layer resin insulating layer 752. Moreover, the metal pad and the resin insulating layer are made of different materials. Therefore, there arises a problem in that the strength of adhesive bonding is unsatisfactorily small. Therefore, warp or asperities of the substrate sometimes occur owing to the difference in the coefficiency of thermal expansion between the package substrate and the mother board or the daughter board under a heat cycle condition in which a high temperature and a low temperature are repeated and which is performed as a reliability test. In the foregoing case, fracture of the interface occurs between the pad 716 and the inter layer resin insulating layer 752. Thus, there arises a problem in that the conductive connecting pin 720 separates from the substrate together with the substrate. When the package substrate is mounted on the mother board by using the conductive connecting pin, displacement between the position of the conductive connecting pin and the socket of the mother board causes stress to be concentrated to the connection portion. As a result, the conductive connecting pin sometimes separates together with the pad. Heat in the high temperature region in the heat cycle and that generated when the IC chip is mounted, sometimes causes the conductive pin to be separated or inclined. Moreover, the electrical connection cannot sometimes be established.
To solve the above-mentioned problems, an object of the present invention is to provide a conductive connecting pin which is free from concentration of stress under heat cycle conditions or during a mounting an electronic component such as an IC chip and a resin package substrate incorporating the conductive connecting pin which cannot easily be peeled and separated and with which the electrical connection can easily be established if the stress is exerted.
The build-up multilayer circuit plate for use as a package substrate has a plane layer for constituting a power supply layer to permit supply of large electric power to the IC chip or a plane layer for constituting an earth layer to reduce noise.
However, the plane layer is connected to the pad which establishes the connection with an external substrate (for example, a daughter board) through a via hole. The plane layer which constitutes the power supply layer for the purpose of supplying electric current from the daughter board portion through the fine via hole is able to supply only limited electric power to the IC chip. Therefore, a satisfactory function cannot be realized. Also the plane layer, which constitutes the earth layer and which is connected to the earth line of the daughter board through the fine via hole having high resistance, cannot satisfactorily reduce noise.
To connect the multilayer printed circuit board for use as the package substrate to the daughter board, the conductive connecting pin must be joined to the pad provided for the multilayer printed circuit board. If a metal pad is provided for the package substrate made of resin, the conductive connecting pin separates together with the pad when stress has been exerted on the conductive connecting pin because the strength of adhesive bonding of each of the two elements is unsatisfactorily low.
To solve the above-mentioned problem, an object of the present invention is to provide a package substrate incorporating a plane layer having a satisfactory function.
Another object of the present invention is to provide a resin package substrate incorporating a plane layer having a satisfactory function and a conductive connecting pin which cannot easily be separated.
On the other hand, the build-up multilayer circuit plate incorporates a BGA constituted by solder or the like in order to establish the connection with an external substrate. Thus, the build-up multilayer circuit plate is mounted on the surface of the external substrate.
When the connection with an external substrate is established through the BGA, too small area of adhesion between the BGA and the solder resist causes the tensile strength to be reduced. As a result, concentration of stress to the BGA or the heat cycle condition employed as the reliable test causes the BGA or a metal layer for holding the BGA to be cracked or broken.
A variety of heat hysteresis experienced with the build-up multilayer circuit board when it is formed, such as drying and hardening of the inter layer resin insulating layer and the solder resist (an organic resin insulating layer), drying occurring after a plated film has been formed and an annealing process causes the substrate to be warped or asperities to occur. The warp and asperities sometimes inhibit the connection between the build-up multilayer circuit plate and the external substrate by using the small BGA.
It might be considered feasible to establish the connection with the external substrate by using PGA as a substitute for the BGA of the build-up multilayer circuit plate. That is, the PGA establishes the electrical connection by inserting a pin into a connection portion of the external substrate. Therefore, the foregoing defect in the connection experienced with the BGA does not occur.
When the PGA is formed, through holes are formed in the substrate by using a drill or a laser beam. Then, the PGA is inserted into the through hole. The build-up multilayer circuit plate incorporates the insulating resin layer which does not contain a reinforcing agent made of glass epoxy resin or the like. Therefore, the strength for supporting the PGA is too small to enlarge the tensile strength. What is worse, plating solution which is used to form a conductive layer in the through hole after the drilling operation has been performed, a variety of heat hysteresis or heat required to melt solder in the through hole to secure the PGA sometimes causes the resin in the inter layer insulating layer to be melted. Thus, the PGA cannot sometimes be disposed.
Since the PGA requires the through hole to be formed, disposition of an electric line in the lower layer is inhibited which is permitted for the BGA. Therefore, the degree of freedom required when the substrate is designed is narrowed excessively.
To solve the foregoing problems, an object of the present invention is to provide a package substrate with which the tensile strength of the PGA is enlarged and the degree of freedom of wiring can be widened and which exhibits a satisfactory connection characteristic with an external substrate.